|MadSci Network: Development|
Dear Dominique, Your question concerns several tissues that are complex, and also diverse in organization among the many species you mentioned. Therefore, a comprehensive answer to your question is beyond the scope of this forum. To begin answering it, I will generalize that the eye is constructed so as to put an image of the outside world on the retina in an appropriate manner, such that any difference in the organization of the retina will be tightly correlated with differences in the organization of the rest of the eye. I will thus limit my discussion to divergences in retinal organization among mammals. To talk about differences between retinas, we must first characterize a “stereotypical” retina. As you know, the retina is located in the back of the eyeball. It is analogous to the film in a camera. That is, the retina is the only light-sensitive tissue in the eye. It is a three dimensional array of more than 50 cell types that receive light from the world, extract various temporal and spatial properties of that light, and transmit the information to the brain. Light entering the retina will be absorbed by photoreceptor cells. These can either be cones (various cone types are more or less sensitive to a given color of light) or rods (of which there is only one type, that is very good at sensing low levels of light). These photoreceptors are connected to bipolar cells, which relay the information to ganglion cells. Ganglion cells send their axons out of the retina, and the bundle of all these axons is the optic nerve. This nerve connects the retina to deep brain structures that are ultimately responsible for visual perception. This is the vertical pathway: photoreceptor to bipolar to ganglion cell. There are also lateral pathways: horizontal cells connect photoreceptor cells together, and amacrine cells mediate connections among bipolar cells, among ganglion cells, and among bipolar and ganglion cells. The pattern of connectivity between a ganglion cell and a pool of photoreceptors (that is, which bipolar, horizontal, and amacrine cells lie between them) determines the ganglion cell’s activity. You can get some fascinating results from this pattern. For example, there are ganglion cells in your eye that react only when objects move in a certain direction. This is a lot to take in, so please visit retina.mgh.harvard.edu to take a look at a picture of the retina. On the main page, click on the picture under "Our research goals." Most of what I described above is shown there both in a real retina and in a helpful schematic. Having sketched a portrait of the retina, we can talk about a difference in its organization among animals. Eyes are specialized either to focus on a point, a line (like the horizon), or some mixture of both. The retina will have a high concentration of cells in an area whose shape reflects this specialization. That is, if the eye is dedicated to looking at a point, the retina will have a roughly circular region of high cell density called the area centralis. If the eye is dedicated to looking at a line, the retina will have an elongated region of high cell density called the visual streak. Animals that live in habitats with little or no view of the horizon tend to have retinas dominated by an area centralis. In most primates and many birds and reptiles, the area centralis is so specialized for high acuity vision that the retina is actually thinned there to reduce the scattering of light entering the retina (light must go through ganglion and bipolar cells before it can be sensed by photoreceptors). This unique area centralis is termed a fovea. In humans, the fovea takes up 2% of the retinal area but accounts for 33% of all ganglion cells. Thus our vision is very sharp, and you can read this from a fair distance away if you look straight on (and the screen falls on your fovea). If you fix your gaze so that the screen is just in the corner of your eye, you will find that you can barely read this at all. That is because the image is not on your fovea, and only a small number of cells are responsible for sensing it. Animals that have visual streaks tend to live on the open plain. This organization is ideal for keeping much of the horizon in view, and thus avoiding predators. However, the acuity at any point on the visual streak tends to be lower than that of the area centralis. Now we can try to make connections between this organization on the retina level with other aspects of the eye. For example, animals with an area centralis tend to have finer eye movements than animals with a visual streak. If accurate vision is limited to a small area, you can see how fine-tuning eye movements so that a moving object can be fixed on that small area is advantageous. Animals with visual streaks do not need eye movements so fine, since they can see a relatively large area at reasonable acuity by keeping their eyes still. Thus, the eye musculature is likely to be different for animals with these two retina types. You can also imagine how pupils might vary between animals with foveas and streaks, admitting light in a characteristic shape. For more information, I would recommend R.W. Rodieck’s _The First Steps in Seeing_ to start. It is published by Sinauer Associates, copyright 1998. For a taste of the staggering diversity of eyes throughout phylogeny, I refer you to an article by Russel D. Fernald in Current Opinion in Neurobiology, 2000, 10:444-450, titled “Evolution of Eyes.” I am certain that tracing references through these two sources will lead you to most of what is known about your topic. Best, Michael Do
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